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La-Based Perovskite-Type Oxide Catalysts for Automotive Exhaust After-Treatment

윤달영 (Dal Young Yoon, 포항공과대학교)

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Since early 1970s, the increased use of automotive engines had brought the serious concerns on urban air quality directly by the emissions included in their exhaust, and indirectly by the more harmful species derived from them via photochemical reactions. Hence, the world-wide automotive emission s...
Since early 1970s, the increased use of automotive engines had brought the serious concerns on urban air quality directly by the emissions included in their exhaust, and indirectly by the more harmful species derived from them via photochemical reactions. Hence, the world-wide automotive emission standards in the US, Europe, Japan, China and Korea have been ever-tightened to further promote the use of cleaner engine and to further reduce the harmful emissions included in engine exhaust. To meet the stringent emission regulations, a variety of the technologies have been developed and employed for the automotive exhaust after-treatment system. Three-way catalysts (TWCs) are the most effective catalytic system for simultaneously removing CO and hydrocarbons (HCs) under the near stoichiometric exhaust gas condition in gasoline engine. Since TWCs are readily deactivated by the exposure of the extremely high exhaust gas temperature, occasionally reaching to 1050 oC and beyond, there has been a strong demand to improve the thermal stability of TWC. On the other hand, the reduction of NOx emitted from the diesel engine has been a primary concern due to its lean operating condition. The NO oxidation over the diesel oxidation catalyst (DOC) for enhancing the deNOx activities of the urea/SCR or LNT system has been thus regarded as one of the important processes in the diesel after-treatment system. The Pt-based catalyst has been regarded as the representative NO oxidation catalyst included in DOC. However, the development of an alternative catalytic system has still been required, mainly due to the high cost and the weak thermal stability of Pt commonly employed. The perovskite-type oxides have a crystal structure in the form of ABO3 and several hundred formulations of perovskites may be readily synthesized by the combination of A- and B-site cations, and partial substitutions of both sites. Moreover, they have been proposed as a thermally durable and highly active catalyst due to their peculiar structural and physicochemical properties. In particular, the clean-up of automotive exhaust emitted from gasoline and diesel engines has been regarded as one of the most potential application areas of the perovskite-type oxide catalysts. However, the catalytic activity of the perovskite-based catalysts developed and proposed until recently may be still required to be further improved for their direct implementation in the gasoline and diesel after-treatment systems, and a new perovskite-based catalyst revealing much higher catalytic performance and stronger thermal stability for TWC and NO oxidation activity is demanded to be developed. In the present study, La-based perovskite-type oxide catalyst has been proposed to be applied to TWC and DOC for gasoline and diesel engine after-treatment systems. A Pd-substituted LaAlO3 perovskite (LaAlPdO3) has successfully demonstrated as a potential and promising TWC to improve the thermal stability of TWC for gasoline engine exhaust. The LaAlO3-based Pd catalysts revealed a superior catalytic activity and stronger thermal stability than the Al2O3-based counterparts. The incorporation of Pd into the structure of LaAlO3 or Al2O3 appears to be a potential and effective way to improve the stability of Pd included upon thermal aging. When the catalyst mileage increases, Pd substituted for Al in the LaAlO3 perovskite structure exhibits a higher catalytic activity and stronger thermal stability than that impregnated on the LaAlO3 surface. The highest decomposition temperature of PdO regarded as the active reaction site for the oxidation reaction of Pd-based TWC and the least transformation of Pd state from PdO to Pd0 during thermal aging had been observed over the Pd-substituted LaAlO3 perovskite, LaAlPdO3, as understood by the XANES, TGA and XPS studies. It indicated that the increased electron density around Pd induced by the electron donation from the more electronegative La may enhance the thermal stability of PdO by suppressing its thermal decomposition to Pd0. The strong interaction of Pd with La appears to contribute to the improvement of the TOFs of the TWC reactions for CO, C3H6, H2 and NO over the Pd-substituted LaAlO3, as evidenced by H2-TPR, DRIFT and TPD studies. The CO and C3H6 oxidation reactions over the LaAlO3-based Pd catalysts are regarded as strongly structure-sensitive reaction, while those over the Al2O3-based counterparts seem to be weakly structure-sensitive. Results of the CO- and C3H6-TPDs over the LaAlO3-based catalysts indicated that their desorption peak temperatures shifted to the lower temperatures as the catalyst mileage increased with the growth of the Pd particle. Then, the TOFs for the CO and C3H6 oxidation reactions have been improved and they may be regarded as strongly structure-sensitive reaction. As the catalyst mileage increased, the TOF for NO over the Pd-containing catalysts examined in the present study apparently increased along with the shift of NO desorption peak to a lower temperature during NO-TPD, indicating the strongly structure-sensitive NO reduction reaction. In addition, LaAlPdO3 revealed the highest TOF for NO reduction regardless of the catalyst mileage which might be understood by the maintenance of the abundant Pd (111) phase on its catalyst surface, as confirmed by DRIFT and TEM studies. The Ag-doped perovskite catalysts (La1-xAgxMnO3) were examined for the catalytic oxidation of NO under the simulated diesel engine exhaust condition for improving the deNOx activity over the urea/SCR or LNT system as well as removing soot collected in the DPF. Compared to the other representative catalysts including La1-xSrxMnO3 and Pt-Pd/Al2O3, La1-xAgxMnO3 revealed a superior NO oxidation activity normalized by their BET surface area with its maximum activity at x = 0.2, confirming the potential of La0.8Ag0.2MnO3 as one of the most promising NO oxidation catalysts whereas the highest NO oxidation activity normalized by the catalyst volume was achieved at x = 0.5, mainly due to its highest BET surface area. The maximum substitution ratio of Ag into LaMnO3 is at around x ? 0.2 and the segregated metallic Ag is formed on the La1-xAgxMnO3 when x ≥ 0.2, as determined by XRD. In addition, oxygen vacancy concentration increased as x increased to 0.2, but it decreased with further increases of x up to 0.8 which is in agreement with the alteration of Mn4+/Mn3+ ratio with respect to x. Hence, the high reactivity of La0.8Ag0.2MnO3 is mainly due to the high oxygen vacancy concentration on its surface produced by the partial substitution of La3+ with Ag+ in LaMnO3. The contribution of metallic Ag particles to the NO oxidation reaction over La1-xAgxMnO3 catalysts might be negligible compared to that of the oxygen vacancies. The IR peak intensities of the mono- and bi-dentate nitrates formed on the oxygen vacancies of perovskite decreased with the increasing catalyst temperature, coupled with the desorption of NO2 from the catalyst surface which may suggest that these nitrate species is the primary reaction intermediates for the NO oxidation reaction over the Ag-doped perovskite catalyst. In addition, a plausible reaction pathway of the NO oxidation over La1-xAgxMnO3 has also been postulated on the basis of the redox process involving the oxygen vacancy, oxygen, NO, NO2 and NO3. Finally, the commercial feasibility of the Ag-doped LaMnO3 perovskite for improving the deNOx performance of NH3/SCR and LNT systems has been investigated. The higher deNOx activities over a dual-bed reactor system comprised of La0.8Ag0.2MnO3 in the front bed and an NH3/SCR or LNT catalyst in the rear bed have been observed compared to those over a single-bed reactor containing commercial deNOx catalyst only, mainly due to the high NO oxidation activity over La0.8Ag0.2MnO3. Based on the present study, the feasibility of the Pd- and Ag-doped La-based perovskite catalyst to be directly applied to gasoline and diesel exhaust after-treatment systems has been extensively determined. The catalytic activity and thermal stability over those catalysts examined with their physicochemical properties may provide a research guideline for developing a new type of the perovskite-based TWC and NO oxidation catalyst for designing a new cost-effective and thermally stable automotive after-treatment system.
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TABLE OF CONTENTS

ABSTACT I
...
TABLE OF CONTENTS

ABSTACT I
TABLE OF CONTENTS VIII
LIST OF TABLES XII
LIST OF FIGURES XV

CHAPTER I. INTRODUCTION 1

CHAPTER II. LITERATURE SURVEY 11
2.1. Automotive exhaust after-treatment technology 11
2.1.1. Three-way catalysis 13
2.1.2. NO oxidation reaction over diesel oxidation catalyst 17
2.2. Application of perovskite-type oxides 19
2.2.1. Three-way catalyst 24
2.2.2. NO oxidation catalyst 30

CHAPTER III. EXPERIMENTAL 41
3.1. Catalyst preparation 41
3.1.1. Pd-based three-way catalyst 41
3.1.2. NO oxidation catalyst 48
3.2. Catalyst washcoating onto the monolith 50
3.3. Reaction System 51
3.3.1 TWC reactor system 51
3.3.2. NO oxidation reactor system 57
3.3.3. Combined deNOx reactor system in NH3/SCR and LNT 64
3.4. Catalyst characterizations 68
3.4.1. ICP-OES 68
3.4.2. BET surface area 68
3.4.3. CO-chemisorption 68
3.4.4. XRD 69
3.4.5. XPS 70
3.4.6. XANES 70
3.4.7. TGA 71
3.4.8. H2-TPR 71
3.4.9. TPD 72
3.4.10. (NO+O2)-TPRD 73
3.4.11. In-situ DRIFT study 73
3.4.12. SEM 74
3.4.13. TEM 74
3.4.14. H2-O2 titration 75

CHAPTER IV. THERMAL STABILITY OF Pd-CONTAINING LaAlO3 PEROVSKITE AS A MODERN TWC 76
4.1. TWC activity over Pd-containing catalysts 76
4.2. Cause of sintering for Pd-containing catalysts 92
4.2.1. Physicochemical properties 92
4.2.2. State of Pd 104
4.2.3. Metal-support interaction 114
4.3. Structure sensitivity of TWC reaction 137

CHAPTER V. NO OXIDATION ACTIVITY OF Ag-DOPED PEROVSKITE CATALYSTS 145
5.1. Effect of partial substitution of A-site in LaMnO3 on NO oxidation
activity over La1-xA’xMnO3 (A’: Ag and Sr) 145
5.2. Role of Ag in Ag-doped perovskite 152
5.2.1. Structural property of Ag-doped perovskite 152
5.2.2. Surface property of Ag-doped perovskite 162
5.3. NO oxidation activity in terms of oxygen vacancies and metallic Ag
sites 180
5.4. DRIFT study of NO oxidation over Ag-doped perovskite 189
5.5. Reactivity and reaction pathways of NO oxidation over Ag-doped
perovskite 199
5.6. Application of La1-xAgxMnO3 to NH3/SCR and LNT systems 203

CHAPTER VI. CONCLUSION AND RECOMMENDATION 207
6.1. Conclusion 207
6.2. Recommendation 212

SUMMARY IN KOREAN 216

REFERENCES 222

APPENDIX 235


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